Recently, renewable energy is expected to be used in view of exhaustion of fossil fuel resources and emission limitation of global greenhouse gas. The renewable energy is generated from various kinds of resources such as sunlight, hydro power, wind power, geothermal heat, tidal power, and biomass, among which, since the sunlight is large in utilizable energy amount and relatively small in geographical constraint as compared with the other renewable energy resources, a technique to efficiently produce utilizable energy from the sunlight is expected to be developed and become widely used in an early stage.
The utilizable energy produced from the sunlight include electric energy produced by a solar cell or a solar thermal turbine, thermal energy produced by collecting solar energy to a heat medium, and stockable fuel energy such as a liquid fuel or hydrogen produced by reducing a substance with the sunlight. While many solar cell techniques and solar heat utilization techniques have been already put to practical use, these techniques are being developed to be improved because energy use efficiency is still low and costs to produce electricity and heat are still high. Furthermore, although the energy in the forms of electricity, heat, and the like can be used to complement a short-term energy fluctuation, there are problems that it is very difficult to complement a long-term fluctuation such as a seasonal fluctuation, and that operation rates of electric power facilities could be lowered due to an increase in energy amount. Meanwhile, to store energy as a substance such as the liquid fuel or hydrogen is extremely important as a technique to efficiently complement the long-term fluctuation and improve the operation rates of the electric power facilities, and it is the indispensable technique to maximally enhance the energy use efficiency and thoroughly reduce a carbon dioxide emission amount in the future.
Stockable fuels can be roughly classified into forms of a liquid fuel such as carbon hydride, a gas fuel such as a biogas or hydrogen, and a solid fuel such as a biomass-derived wood pellet or a metal reduced by the sunlight. Each of these forms have good and bad points such that the liquid fuel is advantageous in view of easiness of infrastructure construction and an energy density, the gas fuel such as hydrogen is advantageous in view of total use efficiency improvement with a fuel cell, and the solid fuel is advantageous in view of stockability and an energy density, among which, a hydrogen producing technique to decompose water by the sunlight especially attracts attention because easily available water can be used as a raw material.
Methods for producing hydrogen using solar energy and water as a raw material include a photolysis method in which platinum is supported on a photocatalyst such as titanium oxide, this material is put into water and a charge is separated in a semiconductor by light irradiation, and a proton is reduced and water is oxidized in an electrolytic solution, a pyrolysis method in which water is directly pyrolyzed at high temperature by use of thermal energy in a high-temperature gas furnace, or water is indirectly pyrolyzed by conjugating metallic redox, a biological method utilizing metabolism of a microbe such as an alga using light, a water electrolysis method in which electricity generated by a solar cell is combined with a hydrogen production device for water electrolysis, and a photovoltaic method in which a hydrogen generation catalyst and an oxygen generation catalyst are supported on a photoelectric conversion material used in a solar cell, and the hydrogen generation catalyst and the oxygen generation catalyst are used in reactions of electron and hole provided by photoelectric conversion. Among them, the photolysis method, the biological method, and the photovoltaic method are regarded as having a possibility to produce a small-size hydrogen production device by integrating a photoelectric conversion part and a hydrogen generation part, but it is believed that the photovoltaic method is one of the most possible techniques to be practically used in view of conversion efficiency of energy from sunlight.
There have been disclosed examples of the hydrogen production device in which photoelectric conversion is integrated with hydrogen generation by the photolysis method or the photovoltaic method. As for the photolysis method, Patent document 1 discloses a device using a photocatalytic electrode of titanium oxide adsorbing a ruthenium complex, a platinum electrode, and redox of iodine or iron. Further, Patent document 2 employs an integral structure by connecting two photocatalytic layers in tandem, connecting a platinum counter electrode, and sandwiching an ion exchanging film therebetween. Meanwhile, as for the photovoltaic method, there is disclosed a concept of a hydrogen production device integrally provided with a photoelectric conversion part, a hydrogen generation part, and the oxygen generation part (Non-patent document 1). According to this document, charge separation is performed by the photoelectric conversion part, and hydrogen generation and oxygen generation are performed by using catalysts respectively corresponding to them. The photoelectric conversion part is made of a material used in a solar cell. For example, according to Non-patent document 2, after charge separation is performed on three silicon p-i-n layers, hydrogen generation is undertaken by a platinum catalyst, and oxygen generation is undertaken by ruthenium oxide. Meanwhile, according to Patent document 3 and Non-patent document 3, an integral hydrogen generation device is produced by laminating a hydrogen generation catalyst (NiFeO) and three silicon p-i-n layers in parallel on a substrate, and supporting an oxygen generation catalyst (Co—Mo) on the silicon layer.